1. Field of the Invention
The present invention relates to a fabrication method of a liquid crystal display device, and more particularly to a method of forming sealants and/or seal patterns for preventing damage during a process of etching substrates.
2. Discussion of the Related Art
As information technologies rapidly develop, flat panel display devices that are slim and light are developed in accordance with the pace of the technology development. A liquid crystal display (LCD) device, which is more researched and developed, is an example of the flat panel display device. The LCD device can produce high quality images at a reasonable cost these days.
A typical LCD device includes an upper substrate, a lower substrate, and a liquid crystal interposed therebetween. The upper and lower substrates respectively have electrodes opposing each other. When an electric field is applied between the electrodes of the upper and lower substrates, molecules of the liquid crystal are aligned according to the electric field. By controlling the above-mentioned electric field, the liquid crystal display device provides various transmittances for light such that an image is displayed.
A fabrication process of the LCD device includes forming an array substrate, forming a color filter substrate, and a cell fabrication process. By the process of forming the array substrate, the array substrate includes thin film transistors, pixel electrodes and other elements. By the process of forming the color filter substrate, the color filter substrate includes color filters and common electrodes. During the cell process, the array substrate and the color filter substrate are aligned and attached to each other so that the common electrodes and the pixel electrodes face each other; the liquid crystal material is injected between the array and color filter substrates; via an injection hole that is subsequently sealed by sealant; and polarizers are applied to the outer surfaces of the array and color filter substrates.
When fabricating the LCD device, a plurality of cells are formed in a large substrate panel in order to increase productivity of the LCD device. In this case, the large substrate panel is cut into individual cells during the cell process after aligning and attaching the color filter substrate to the array substrate. The detailed explanation of the cell process will be presented with reference to FIG. 1.
FIG. 1 is a flow chart illustrating a fabrication process of a liquid crystal cell according to a related art.
At step st1 an array substrate, (often referred to as a thin film transistor substrate) including an array matrix of thin film transistors and pixel electrodes is formed and a color filter substrate, including a color filter layer and a common electrode is formed. The thin film transistor substrate is formed by repeatedly depositing and patterning thin films. Patterning the thin films using masks is a critical process in the formation of the thin film transistor substrate. How to decrease the steps in mask processes during the patterning process has become a major issue that has been enthusiastically researched and developed in order to reduce a manufacturing cost. The color filter substrate includes red, green and blue color filters, and a black matrix that isolates each color filter and prevents light leakage in a portion except for pixel regions. The color filter substrate also includes a common electrode. There are many methods of forming the color filters, such as a dyeing method, a pigment dispersion method, a staining method, a printing method, etc. Among these methods, the pigment dispersion method is mainly used in forming the color filters.
In step st2, orientation films are formed on the thin film transistor and color filter substrates to determine an initial alignment direction of the liquid crystal molecules. Formation of the orientation film includes depositing a polymeric thin film and subsequently performing a uniform rubbing process. The rubbing process determines an initial alignment direction and supplies the normal operation of the liquid crystal layer and the uniform display characteristic of the LCD device. The rubbing process easily determines and controls the pretilt angle of the liquid crystal molecules, provides a stable alignment of the liquid crystal molecules, and is adequate to produce the LCD devices on a large scale. Typically, an organic material of the polyimide series is used as the orientation film. The rubbing method includes rubbing the orientation film along the specific direction with a rubbing cloth, thereby aligning the liquid crystal molecules along the rubbing direction.
In step st3, seal patterns are formed in one of the thin film transistor and color filter substrates. In the liquid crystal cell, the seal pattern serves two functions: forming a gap for liquid crystal material injection and confining the injected liquid crystal material. The seal patterns are respectively formed in an area for each cell. The seal patterning process forms a desired pattern by application of a thermosetting resin. A screen-printing method using a screen mask or a seal-dispenser method using a dispenser is typically used for this process. Although the screen-printing method is used more frequently than the seal-dispenser method for the shake of convenience in the manufacturing process, the screen-printing method may cause defects because the screen mask for printing contacts the orientation film. Further, using the screen mask it is difficult to cover the entire substrate as the substrate becomes larger. Therefore, the seal-dispenser method has been adopted more and more recently.
In step st4, a spacer is sprayed on one of the thin film transistor and color filter substrates. The size of the spacer used in the liquid crystal cell maintains a precise and uniform gap between the thin film transistor substrate and the color filter substrate. Accordingly, the spacers are uniformly sprayed on the lower substrate. The method of spraying the spacer includes a wet-spraying method and a dry-spraying method. In the wet-spraying method, the spacer is mixed with alcohol and then sprayed on the substrate. In the dry-spraying method, only the spacer is sprayed on the substrate without any mixture. The dry-spraying method utilizes either one or both of a high speed gas stream manner and an electrostatic dispersion manner. In high speed gas stream manner a predetermined amount of spacer particle is electrified by friction through pipes, and the spacer particle is then expelled from a nozzle to the substrate. In addition, in the electrostatic dispersion manner a spacer particle is expelled from a high-voltage-applied nozzle to a grounded substrate.
In step st5, the thin film transistor and the color filter substrate are aligned and attached by a thermal-hardening process under pressure. The alignment margin between the color filter substrate and the thin film transistor substrate is determined by the device design, and accuracy within a few micrometers is generally required. If the alignment margin is exceeded, the liquid crystal cell will not operate adequately due to light leakage.
In step st6, the attached substrate is divided into unit cells. Generally, a plurality of unit cells are formed on a large sized glass substrate and then divided through a cutting process. In some LCD device fabrication processes, the unit cells are separated after simultaneous injection of the liquid crystal material into the plurality of unit cells. However, injection of liquid crystal material is commonly performed after a large sized liquid crystal substrate is cut into unit cells due to an increased unit cell size. The cell cutting process includes a scribe process that forms cutting lines on a surface of the substrate using a diamond pen, the hardness of which is higher than that of the glass substrate, and a break process that divides the substrate by force.
In step st7, a liquid crystal material is injected into the unit cells. A typical unit cell has a size of several hundred square centimeters with a gap of several micrometers. Accordingly, a vacuum injection method using pressure difference between the interior and exterior of the unit cell is commonly used as an effective injection method. When the liquid crystal material is interposed in the unit cells, air bubbles may exist in the liquid crystal layer and these air bubbles may cause the defects. Thus, the liquid crystal material should be left under a vacuum condition for a sufficiently long time to eliminate the air bubbles from the liquid crystal (i.e., a de-aeration process).
After injecting the liquid crystal material into the unit cell, an injection hole through which the liquid crystal material was injected is sealed to prevent the liquid crystal material from leaking from the unit cell. Sealing the injection hole includes applying a thermosetting material to the injection hole using a dispenser and then hardening the applied thermosetting material.
After manufacturing the liquid crystal cell as described before, polarizers are attached in the outer parts of the liquid crystal cell and then a driving circuit is connected to the liquid crystal cell, thereby forming a liquid crystal display panel.
The liquid crystal display panel fabricated by the above-described process is typically used as a monitor of the laptop computer. Since the laptop computer should be portable to carry from one place to another, the laptop computer requires a light weight with a small bulk. Thus, the liquid crystal display panel used for the laptop computer must be thin and light.
In order to decrease the thickness and weight of the liquid crystal display panel, the outer surfaces of the attached substrate (the color filter substrate and the thin film transistor substrate) are substantially etched after the attachment process (the step st5 of FIG. 1) and before the cell-cutting process (the step st6 of FIG. 1).
FIG. 2 show a flow chart illustrating an etch process of the attached substrate.
In step st11, alien substances that may have adhered to the outer surfaces of the color filter and thin film transistor substrates are removed from the attached substrate before surface-etching the attached substrates. If these alien substances remain on the outer surfaces of the attached substrate before the surface etching, the etching process is not properly performed around the alien substance, causing etching defects. Thus, the outer surfaces of the attached substrate are rough and rugged. The rough surfaces let the incoming or outgoing light to scattered-reflect and refract. Namely, they cause the scattered reflection and refraction of the light. Thus, the alien substances should be removed from the surfaces of the attached substrate. As a cleaner for removing the alien substances, isopropyl alcohol or deionized water is applied to the outer surface of the attached substrate.
In step st12, the attached substrate is etched. A glass substrate is generally used as a substrate for the LCD device. Since the glass substrate includes about 60% of silicon oxide (SiO2), an HF (hydrogen fluoride) solution is used as an etchant for etching the attached substrate.
Next, the etched substrate is rinsed (step st13), and then a drying process for drying the etched substrate is performed (step st14).
After etching the outer surfaces of the attached substrate, the cell-cutting process (step st6 of FIG. 1) and the injection of liquid crystal material (step st7 of FIG. 1) are sequentially performed. However, during the etching process (step st12 of FIG. 2), the etchant permeates and spreads between two substrates so that the seal patterns are damaged and the array matrix elements are eroded. Thus, in order to overcome these problems, other seal patterns having an advantageous configuration are suggested.
FIG. 3 is a plan view illustrating a seal pattern structure according to a related art, and FIG. 4 is an enlarged plan view illustrating a portion A of FIG. 3.
As shown in FIGS. 3 and 4, a plurality of seal patterns are formed on a substrate 10. The seal patterns are classified as a main seal pattern 20 that defines the unit cell and confines the injected liquid crystal material, a first auxiliary seal pattern 30 that surrounds the main seal pattern 20, and a second auxiliary seal pattern 40 that surrounds the first auxiliary seal pattern 30 and is positioned in periphery of the substrate 10. The first and second auxiliary seal patterns 30 and 40 each have at least one opening T, respectively. Furthermore, a plurality of third auxiliary seal patterns 50 are formed between the first and second auxiliary seal patterns 30 and 40 near the opening T of the second auxiliary seal pattern 40.
FIG. 4 is an enlarged plan view of a portion A of FIG. 3 and a detailed illustration of the third auxiliary seal patterns 50. As shown, the third auxiliary seal patterns 50 are formed parallel to one another and spaced apart from each other. One end of the third auxiliary seal patterns 50 contacts the first auxiliary seal pattern 30 or the second auxiliary seal pattern 40, and the other ends of the third auxiliary seal pattern 50 are spaced apart from the first auxiliary seal pattern 30 or the second auxiliary seal pattern 40. The third seal patterns 50 contacting the first auxiliary seal pattern 30 are alternately disposed between the third seal patterns 50 contacting the second auxiliary seal patterns 40.
Therefore, in FIGS. 3 and 4, the inside of the first to third auxiliary seal patterns 30, 40 and 50 is not isolated from the outside, and the air can communicates through the openings T of the first and second auxiliary seal patterns 30 and 40. Namely, the air inside the seal patterns can flow out though the openings T and through the third seal patterns 50 when attaching the upper and lower substrates to each other.
However, in practice, the auxiliary seal patterns are too complex for the air to flow out. Thus, when the substrate having the seal patterns is attached to the other substrate using a hot press process, the air inside the seal patterns is blocked by the seal patterns and is blocked from coming out of the inside. The air kept inside the seal patterns causes breakage of the seal patterns. This breakage of the seal patterns allows the etchant to come into contact with the main seal pattern 20 of the liquid crystal cell when the outer surfaces of the attached substrate are etched. Thus, the main seal pattern 20 can be damaged and etchant may leak into the cell gap.